The present invention relates to proof masses for accelerometers, and has particular relation to such proof masses which have been rendered resistant to static electricity.
An accelerometer consists of a proof mass supported equidistant between two stators. Each stator includes a permanent magnet. The proof mass includes an electromagnet, or a pair of electromagnets wired in series, one on each side of the proof mass. When the proof mass is forced by acceleration toward one of the stators, additional current is supplied to the electromagnet to keep it exactly halfway between the stators. The amount of the current supplied to the electromagnet in therefore indicative of the acceleration.
The proof mass is kept exactly halfway between the stators by plating opposite sides of the proof mass with gold, or another suitable conductor. The stators are wired together, and an ac voltage is applied to the stators, with respect to ground. The plate on each side of the proof mass interacts with the stator closest to it to form a capacitor, and the ac voltage applied to the stators causes a current to flow through each plate. If the proof mass moves toward one stator, the capacitance on that side increases, and increases the current with it. On the other side, the capacitance and current decrease. These two currents can be applied to a differential current amplifier, the output of which is applied to the coils so as to keep the proof mass at the midpoint position.
The proof mass is designed to respond only to acceleration, gravitational or otherwise. Electromagnetic forces, in particular, should have no affect on the proof mass. With magnetic forces, this can be accomplished by making the proof mass of non-magnetic materials, such as fused silica for the non-conductive portions and gold for the conductive portions. However, with fused silica and other non-conductive materials, electrostatic charges are inevitable, especially between the proof mass proper and the metalic stators. This electrostatic force cannot be differentiated from an ordinary acceleration, and produces a bias in the instrument.
Further, when the accelerometer is turned off, the proof mass rests against one or the other of the stators. In order to avoid shorting between the pickoff plate and the stator against which the proof mass rests, polyimide buttons are placed on the stator, insulating the pickoff plate from the stator. However, if one of the polyimide buttons makes contact with the fused silica, it will deposit an electrostatic charge on the fused silica, thereby restarting a long discharge period. It is possible to simply out wait the electrostatic charge, and to take whatever time is necessary to allow it to discharge. However, this ties up valuable equipment and is not cost effective in production.